I have been using the Digital River companies RegNow and SWreg for about 14 years. They give good service, and are fairly easy to set up. In that time I have never had any transaction from my websites compromised, their security is excellent.
The one thing that prevents a 5 star rating is their customer support. Whenever I've sent a "Contact Us" support request, I have not received an answer. They must think that if they ignore you, you'll eventually find the answer. I usually have, but after a lot of wasted time and frustration that could have been averted by the courtesy of a simple answer from them.